Please note that Sitefinity will support URLs without extensions if you set wildcard application mappings in IIS. For details, you can refer to this section of the User Manual: Configuring IIS > Wildcard Application Mappings.
I would recommend using another approach. First of all, you should set the About Us page as page group in Sitefinity and then add a Default.aspx page (set as normal page) as its first subpage. The page group will redirect to its first subpage.
The next step would be configuring the IIS to handle such kind of environment.
In the Configuring IIS > Extensionless URLs section of the User Manual, you can find the detailed instructions about setting up IIS to handle such requests.
Thank you for your time.
the Telerik team